This invention relates to engine exhaust driven turbochargers and more particularly to a turbocharger rotor having alignment couplings and a fastener rod joining compressor and turbine wheels with a connecting shaft.
It is known in the art relating to exhaust driven engine turbochargers to provide a rotor including a turbine wheel and a compressor wheel connected by a shaft for rotation together about an axis. In some cases, the shaft is formed as an extension of the turbine wheel. Separate shaft and wheel components may be welded together before final machining. Alternatively, a steel shaft may be connected to the turbine and to the compressor wheel by separate connecting means. Commonly, the impeller or compressor wheel is made of aluminum alloy to minimize the rotating mass.
Various types of connecting means have been provided for aligning and connecting the wheels and the shaft for axial rotation. Where the connecting means extend through the compressor wheel and clamp the wheel in compression against the shaft, the design should avoid excessive variations in clamping load due to differential thermal growth and the effects of centrifugal force on the steel and aluminum during varying operating and stationary conditions. The means for connecting the compressor impeller wheel and the turbine wheel to the shaft are also important because the rotor must be disassembled after balancing in order to assemble the rotor into the turbocharger. Upon reassembly of the rotor, the repeat balance must preserve the original balance as far as possible without actually rebalancing the rotor in the turbocharger assembly. Connecting means that allow separation and reassembly of the components without changing the balance are therefore desired.
The present invention provides a rotor including a turbine wheel and a compressor wheel connected by a shaft for rotation together about an axis. Novel connecting means extend between the compressor and turbine wheels and limit the clamp load, or retaining force, variation applied to the compressor wheel under varying thermal expansion conditions experienced during operation and shutdown. The connecting means also provide for coaxially aligning or centering the compressor and turbine wheels on the axis of the connecting shaft with the capability of simple and repeatable reassembly.
The connecting means include a single long fastener rod, such as a stud or bolt, which extends through both the compressor wheel and the connecting shaft to engage the turbine wheel and place both the compressor wheel and the connecting shaft in compression. Preferably the fastener rod is threaded into the turbine wheel and carries a nut or head that clamps the compressor wheel and shaft in assembly with the turbine wheel. Optionally, the fastener rod could also extend through the turbine wheel and be secured to the turbine wheel by a nut or head.
The connecting means also include first and second joints between the shaft and the compressor wheel at one end and the turbine wheel at the other end. The joints are configured to maintain coaxial alignment of the compressor and turbine wheels with the shaft while providing high axial and bending stiffness and torque transmitting capability. Various forms of joints could be provided to meet these requirements. Examples include piloted shoulders and polygon connections as well as toothed couplings, among others. A presently preferred embodiment uses toothed couplings with so-called CURVIC(trademark) coupling teeth.
Another preferred feature of the invention includes use of a steel adapter which is press fitted onto a stub of the aluminum alloy compressor wheel to provide a joint material similar to that of the connecting shaft. The adapter may also provide an oil sealing surface. A similar adapter may also be provided on the turbine wheel if desired.
The shaft may include one or more radial thrust surfaces preferably located inboard of associated bearing journals to limit oil sealing requirements. The thrust surfaces preferably face outward and are formed on flanges integral with the shaft.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.